Implantable neurostimulators having reduced pocket stimulation

ABSTRACT

Neurostimulators and methods of using neurostimulators are provided. The neurostimulator is implanted within a tissue pocket of a patient, and electrical energy is conveyed from the neurostimulator to stimulate a target tissue site remote from the tissue pocket. The neurostimulator has a case with which one or more electrodes are associated. The electrical energy is returned to the electrode(s) in a manner that prevents, or at least reduces, pocket stimulation that may otherwise occur due to the return of electrical energy to the case of the neurostimulator.

RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 12/629,814, filed on Dec. 2, 2009, which claims the benefit under 35U.S.C. §119 of U.S. Provisional Patent Application Ser. No. 61/119,662,filed Dec. 3, 2008, which is incorporated hereby reference in itsentirety.

FIELD OF THE INVENTION

The present inventions relate to tissue stimulation systems, and moreparticularly, to systems and methods for preventing or reducinginadvertent stimulation of tissue adjacent an implantableneurostimulator.

BACKGROUND OF THE INVENTION

Implantable neurostimulation systems have proven therapeutic in a widevariety of diseases and disorders. Pacemakers and Implantable CardiacDefibrillators (ICDs) have proven highly effective in the treatment of anumber of cardiac conditions (e.g., arrhythmias). Spinal CordStimulation (SCS) systems have long been accepted as a therapeuticmodality for the treatment of chronic pain syndromes, and theapplication of tissue stimulation has begun to expand to additionalapplications, such as angina pectoris and incontinence. Deep BrainStimulation (DBS) has also been applied therapeutically for well over adecade for the treatment of refractory Parkinson's Disease, and DBS hasalso recently been applied in additional areas, such as essential tremorand epilepsy. Further, in recent investigations, Peripheral NerveStimulation (PNS) systems have demonstrated efficacy in the treatment ofchronic pain syndromes and incontinence, and a number of additionalapplications are currently under investigation. Furthermore, FunctionalElectrical Stimulation (FES) systems such as the Freehand system byNeuroControl (Cleveland, Ohio) have been applied to restore somefunctionality to paralyzed extremities in spinal cord injury patients.

Each of these implantable neurostimulation systems typically includesone or more electrode carrying stimulation leads, which are implanted atthe desired stimulation site, and a neurostimulator (i.e., animplantable pulse generator (IPG)) implanted remotely from thestimulation site, but coupled either directly to the stimulation lead(s)or indirectly to the stimulation lead(s) via a lead extension. Forexample, in the context of SCS, the electrode lead(s) are typicallyimplanted along the dura of the spinal cord, with the electrode lead(s)exiting the spinal column, where they can generally be coupled to one ormore electrode lead extensions. The electrode lead extension(s), inturn, are typically tunneled around the torso of the patient to asubcutaneous pocket (typically in the chest or abdomen) where theneurostimulator is implanted.

The neurostimulation system may further comprise a handheld patientprogrammer to remotely instruct the neurostimulator to generateelectrical stimulation pulses in accordance with selected stimulationparameters. The handheld programmer, which may take the form of a remotecontrol (RC) may, itself, be programmed by a clinician, for example, byusing a clinician's programmer (CP), which typically includes a generalpurpose computer, such as a laptop, with a programming software packageinstalled thereon.

Thus, electrical pulses can be delivered from the neurostimulator to thestimulation electrode(s) to stimulate or activate a volume of tissue. Inparticular, electrical energy conveyed between at least one cathodicelectrode and at least one anodic electrodes creates an electricalfield, which when strong enough, depolarizes (or “stimulates”) theneurons beyond a threshold level, thereby inducing the firing of actionpotentials (APs) that propagate along the neural fibers.

Electrical pulses may be delivered from the neurostimulator to thestimulator electrode(s) in accordance with a set of stimulationparameters and provide the desired efficacious therapy to the patient. Atypical stimulation parameter set may include the electrodes that aresourcing (anodes) or returning (cathodes) the stimulation current at anygiven time, as well as the amplitude, duration, and rate of thestimulation pulses.

Stimulation energy may be delivered to the electrodes during and afterthe lead placement process in order to verify that the electrodes arestimulating the target neural elements and to formulate the mosteffective stimulation regimen. The regimen will dictate which of theelectrodes are sourcing current pulses (anodes) and which of theelectrodes are sinking current pulses (cathodes) at any given time, aswell as the magnitude, duration, and rate of the electrical currentpulses. The stimulation regimen will typically be one that providesstimulation energy to all of the target tissue that must be stimulatedin order to provide the therapeutic benefit, yet minimizes the volume ofnon-target tissue that is stimulated. In the case of SCS, such atherapeutic benefit is “paresthesia,” i.e., a tingling sensation that iseffected by the electrical stimuli applied through the electrodes.

Electrical energy may be transmitted to the tissue in a monopolar ormultipolar (e.g., bipolar, tripolar, etc.) fashion. Monopolar deliveryoccurs when a selected one or more of the lead electrodes is activatedalong with the case of the neurostimulator, so that electrical energy istransmitted between the selected electrode and the case. Bipolardelivery occurs when two of the lead electrodes are activated as anodeand cathode, so that electrical energy is transmitted between theselected electrodes. Tripolar delivery occurs when three of the leadelectrodes are activated, two as anodes and the remaining one as acathode, or two as cathodes and the remaining one as an anode.

. In the context of SCS, the neurostimulator case is used as a cathodicreturn electrode, and the lead electrodes are used as anodic stimulatingelectrodes. The neurostimulator case is selected as the cathodic returnelectrode, because it is relatively far away from the stimulation site,and because it has a large surface area, resulting in relatively smallcurrent densities.

This pocket stimulation problem is exacerbated when microstimulators areused. A “microstimulator” is an implantable neurostimulator in which thebody or case of the device is compact (typically on the order of a fewmillimeters is diameter by several millimeters to a few centimeters inlength). For example, the Bion® microstimulator (manufactured anddistributed by Boston Scientific Neuromodulation Corporation) is a tinyfraction of the size of the Precision® IPG. Typically, the cases of themicrostimulators carry electrodes for producing the desired electricalstimulation current. Microstimulators of this type (i.e.,microstimulators with leadless electrodes) are implanted proximate tothe target tissue to allow the stimulation current to stimulate thetarget tissue to provide therapy for a wide variety of conditions anddisorders. In these cases, it is, of course, desired for the pocket inwhich the microstimulator is implanted to be stimulated.

However, it may sometimes be desirable to connect one or more short,flexible stimulation leads to a microstimulator, as described in U.S.patent application Ser. No. 09/624,120, filed Jul. 24, 2000, which isexpressly incorporated herein by reference. The use of such leads maypermit electrical stimulation to be directed more locally to targettissue a short distance from the microstimulator, while allowing themicrostimulator to be located in a more surgically convenient site. Inthis case, stimulation of the implantation pocket is undesirable.

Because the case of a microstimulator is relatively small, the currentdensity on the surface of the case may be quite high when themicrostimulator is operated in a monopolar mode. For example, thesurface area on the case of a Precision® IPG is 3882 mm², whereas thesurface area of the anodic surface of the Bion® microstimulator isapproximately 50 mm². If this anodic surface were used with a leadedBion® microstimulator, undesired and perhaps annoying or painfulstimulation in the implantation pocket might be expected.

Attempts have been made to prevent or, at least reduce, inadvertentpocket stimulation when operating a neurostimulator in a monopolar mode.For example, it is known to coat a portion of the neurostimulator case(e.g., the edges where current density is the greatest) with aninsulative material in order to reduce pocket stimulation (see ToshimiYajima, et al. “Effects of Muscle Potential Depression and MuscleStimulation Caused by Different Insulation Coating Configurations onCardiac Pacemakers: The Use of Insulative Coatings to Try to ReducePocket Stimulation,” J Artif Organs (2005) 8:47-50; Davies T, “DoPermanent Pacemakers Need an Insulative Coating? Results of ProspectiveRandomized Double-Blind Study,” Pacing Clin. Electrophysiol. 1997October; 20(10 Pt 1):2394-7). However, coating a portion of theneurostimulator case necessarily increases the current density of theuncoated portions, thereby potentially increasing the chance that pocketstimulation will occur adjacent these higher current density sections.Furthermore, new edges are created between the coated and uncoatedportions of the neurostimulator case, thereby creating higher currentdensities at these new edges.

There, thus, remains a need to provide an improved neurostimulator andtechnique that prevents, or at least, minimizes inadvertent pocketstimulation.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present inventions, a method ofproviding therapy using a neurostimulator implanted within a tissuepocket of a patient is provide. The neurostimulator has a case and oneor more electrodes associated with the case. The electrode(s) may format least a portion of the case or may be separate elements that aredisposed on the case. The method comprises conveying electrical energyfrom the neurostimulator (e.g., via at least one stimulation lead),thereby stimulating a target tissue site remote from the tissue pocket,and returning the electrical energy to the electrode(s) and at least oneexternal lead connected to the case, which may be electrically coupleddirectly to at least one of the one or more electrodes.

In one method, the electrical energy is returned to the electrode(s) andthe external lead(s) to prevent, or at least reduce, inadvertentstimulation of the tissue pocket that would otherwise occur. If astimulation lead is used, it may either comprise the external lead(s) ormay be separate from the external lead(s). In another method, theexternal lead(s) comprises a plurality of electrodes, in which case, theelectrodes can be selectively activated to return the electrical energy.The electrical energy returned to the electrodes can be independentlyadjusted, e.g., by selecting a fractionalized electrical current for theactivated electrodes. In another method, the external lead(s) comprisesan expandable electrode that will return the electrical energy, in whichcase, the expandable electrode can be expanded to increase its surfacearea and implanted into the patient.

In accordance with another aspect of the present inventions, aneurostimulator is provided. The neurostimulator has a case and aplurality of electrodes associated with the case. The electrodes mayform at least a portion of the case or may be separate elements that aredisposed on the case. The neurostimulator further comprises one or moreconnectors configured for being coupled to one or more stimulationleads, at least one external lead electrically coupled directly to atleast one of the electrode(s), and stimulation circuitry containedwithin the case.

The stimulation circuitry is configured for conveying electrical energyto the stimulation lead(s), and returning the electrical energy to theelectrode(s) and the external lead(s). The stimulation lead(s) mayeither comprise the external lead(s) or be separate from the externallead(s). In one embodiment, the external lead(s) comprises an expandableelectrode configured to return the electrical energy, and configured tobe manipulated to increase a surface area of the expandable electrode.The expandable electrode may comprises a plurality of blades, and may beexpandable, e.g., in the radial direction or the lateral direction.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is plan view of one embodiment of a tissue stimulation systemarranged in accordance with the present inventions;

FIG. 2 is a perspective view of a microstimulator used in the tissuestimulation system of FIG. 1;

FIG. 3 is a block diagram of the electronic circuitry contained withinthe microstimulator of FIG. 2;

FIG. 4 is a plot of strength-duration curves for an exemplary smallnerve fiber and an exemplary large nerve fiber;

FIG. 5 is a perspective view of one embodiment of the microstimulator ofFIG. 2 that can be operated to reduce pocket stimulation;

FIG. 6 is a plot of a stimulation pulse and temporally spaced returnpulses that can be generated by the microstimulator of FIG. 5 to reducethe pocket stimulation;

FIG. 7 is a perspective view of another embodiment of themicrostimulator of FIG. 2 that can be operated to reduce pocketstimulation;

FIG. 8 is a perspective view of still another embodiment of themicrostimulator of FIG. 2 that can be operated to reduce pocketstimulation;

FIGS. 9A-9C are perspective views of other embodiments of themicrostimulator of FIG. 2 that can be operated to reduce pocketstimulation; and

FIGS. 10A-10B are perspective views of still other embodiments of themicrostimulator of FIG. 2 that can be operated to reduce pocketstimulation.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Turning first to FIG. 1, an exemplary tissue stimulation system 10generally includes a neurostimulator, and in particular, amicrostimulator 12, an implantable stimulation lead 14, an externalcontrol device in the form of a remote controller RC 16, and aclinician's programmer (CP) 18. The stimulation system 10 may alsoinclude an external trial stimulator (not shown) for testing the effectsof the stimulation prior to implantation of the microstimulator 12, andan external charger 20 for recharging the microstimulator 12.

It should be noted that, although the neurostimulator is described asbeing a microstimulator herein, the present invention may be used withused with any type of implantable electrical circuitry used to stimulatetissue where it is desirable to prevent pocket stimulation. It shouldalso be noted that, although the microstimulator 12 is described hereinas being battery-powered, the microstimulator 12 alternatively beradio-frequency (RF)-controlled. Ultimately, the architecture of themicrostimulator 12 will depend on the context in which it is intended tobe used.

For example, for some patients, the use of a stimulator for only a fewhours per day or week will improve the symptomatology of the ailment orailments suffered by the patient. In such patients, RF-controlleddevices provide an adequate amount of stimulation if usedintermittently, e.g., for only a few hours per day, to greatly decreasethe incidence of the symptoms. For many other patients, however, acontinuous or intermittent stimulation throughout the day is needed.These patients may best utilize a stimulator that has a self-containedpower source sufficient to deliver stimulation for several hours andthat can be recharged repeatedly, if necessary. Thus, the use of astimulator with a rechargeable battery, thus, provides these patientsthe portability needed to free the patient from reliance on RF powerdelivery.

For purposes of the specification, it is sufficient to note thatRF-controlled stimulators receive power and control signals from anextracorporeal device via inductive coupling of a modulated RF field.Battery-powered stimulators incorporate a power source within the deviceitself, but rely on RF control, inductive linking, or the like toprogram stimulus sequences and to recharge the power source, whenneeded. Whether RF-controlled or battery-powered the implantedstimulator may be commanded to generate pulsed electrical stimulationenergy (i.e., a temporal series of electrical pulses) in accordance witha set of stimulation parameters, including pulse amplitude, pulse width,pulse rate, etc. Further details discussing battery-powered stimulatorsare disclosed in PCT Publication WO 98/37926, WO 98/43700, and WO98/43701, and U.S. Patent Publication No. 2008/0097529, which areexpressly incorporated herein by reference.

Thus, once the microstimulator 12 and stimulation lead 14 are implanted,the RC 16 may be used to telemetrically control the IPG 14 via abi-directional RF communications link 22. Such control allows themicrostimulator 12 to be turned on or off and to be programmed withdifferent stimulation parameter sets. The RC 16 may also be operated tomodify the programmed stimulation parameters to actively control thecharacteristics of the electrical stimulation energy output by themicrostimulator 12.

The CP 18 provides clinician detailed stimulation parameters forprogramming the microstimulator 12 in the operating room and infollow-up sessions. The CP 18 may perform this function by indirectlycommunicating with the microstimulator 12, through the RC 16, via an IRcommunications link 24. Alternatively, the CP 18 may directlycommunicate with the microstimulator 12 via an RF communications link(not shown). The external charger 20 is a portable device used totranscutaneously charge the microstimulator 12 via an inductive link 26.Once the microstimulator 12 has been programmed, and its power sourcehas been charged by the external charger 20 or otherwise replenished,the microstimulator 12 may function as programmed without the RC 16 orCP 18 being present.

The stimulation lead 14 carries a plurality of electrodes 28 arranged inan array. In the illustrated embodiment, the stimulation lead 14 is aflexible percutaneous lead, and to this end, the electrodes 28 arearranged in-line along the stimulation lead 14. In the illustratedembodiment, the stimulation lead 14 includes four collinear electrodes28, but may include as little as two electrodes 28 and as many assixteen or more electrodes 28. In alternative embodiments, theelectrodes 28 may be arranged in a two-dimensional pattern on a singlepaddle lead. The electrodes 28 may be composed of a noble or refractorymetal or compound, such as platinum, iridium, tantalum, titanium,titanium nitride, niobium, or alloys thereof, in order to avoidcorrosion or electrolysis, which could damage the surrounding tissuesand/or stimulation lead 14.

The use of the stimulation lead 14 permits electrical stimulation to bedirected more locally to targeted tissue sites a short distance from thetissue pocket in which the microstimulator 14 will be implanted. In oneembodiment, the leads are no longer than 120 mm. The stimulation lead 14is preferably less than 5 mm in diameter, and more preferably less than1.5 mm in diameter. The stimulation lead 14 further comprises a proximalconnector (not shown) and wires (not shown) electrically connecting theelectrodes 28 to the proximal connector. Further details regarding theuse of stimulation leads with microstimulators are disclosed in U.S.patent application Ser. No. 09/624,130, filed Jul. 24, 2000, which isexpressly incorporated herein by reference.

As shown in FIG. 1, a single microstimulator 12 and stimulation lead 14are implanted under the skin 29 of the patient, with the microstimulator12 disposed into a subcutaneous pocket 30, and the electrode array 28 ofthe stimulation lead 14 disposed at a target tissue site 32 remote fromthe subcutaneous pocket 30. Nerve bundles at the target tissue site 32may carry somatic sensor axons supplying receptors in skin and muscleand somatic motor axons supplying skeletal muscle, as well as autonomicaxons supplying visceral and glandular structures and smooth muscle. Inalternative techniques, multiple microstimulators 12 with respectivestimulation leads 14 or a single microstimulator 12 with multiplestimulation leads 14 may be implanted to achieve greater stimulation ofthe target tissue site 32.

The microstimulator 12 may be implanted into the patient with a surgicalinsertion tool specifically designed for this purpose, as described inU.S. Pat. No. 6,582,441, which is expressly incorporated herein byreference, or may be placed, for example, via a small incision andthrough a small cannula. Alternatively, the microstimulator 12 may beimplanted via conventional surgical methods, or may be inserted usingother endoscopic or laparoscopic techniques. A more complicated surgicalprocedure may be required for the purposes of fixing the microstimulator12 in place. The stimulation lead 14 may be implanted into the patientusing suitable means, such as an endoscope or laparoscope, and mated tothe microstimulator 12.

Referring now to FIG. 2, the microstimulator 12 includes a case 34 andelectronic circuitry 38 (shown in FIG. 3) contained within the case 34.A preferred microstimulator 12 is sufficiently small to permit itsplacement near structures with very little discomfort. As such, the case34 may have an area equal to or less than 1000 mm², and preferably, lessthan 200 mm², and may have a diameter less than 5 mm and a length lessthan 35 mm. The shape of the microstimulator 12 may be determined by thestructure in which it will be implanted, the surrounding area, and themethod of implantation. In the illustrated embodiment, the case 34 takesthe form of a narrow, elongated body with an oblong cross-section, butother shapes, such as rounded cylinders, spheres, disks, and helicalstructures, are possible. The outer case 34 is composed of anelectrically conductive, biocompatible material, such as titanium, andforms a hermetically sealed compartment wherein the internal electronicsare protected from the body tissue and fluids. In the illustratedembodiment, the case 34 or a portion thereof may serve as an electrode.

The microstimulator 12 further includes a connector 36 to which theproximal end of the stimulation lead 14 can be mated, therebyelectrically coupling the electronic circuitry to the electrode array28. In particular, as shown in FIG. 3, the electronic circuitry 38 isalso electrically coupled to the connector 36 via one or more outputterminals 40 (in this case, eight terminals for the eight electrodes),and electrically coupled to the case 34 via one or more electricalterminals 42 (in this case, a single terminal), so that the electricalenergy can be conveyed to the electrodes 28, and thus, the tissue targetsite 32, and then returned at the case 34.

The electronic circuitry 38 further includes analog stimulationcircuitry 44 in the form of pulse generation circuitry that delivers theelectrical stimulation energy in the form of a pulsed electricalwaveform to the electrode array 28 in accordance with a set ofstimulation parameters. Such stimulation parameters may compriseelectrode combinations, which define the electrodes (including the case34) that are activated as anodes (positive), cathodes (negative), andturned off (zero), and electrical pulse parameters, which define thepulse amplitude (measured in milliamps or volts depending on whether themicrostimulator 12 supplies constant current or constant voltage to theelectrode array 28), pulse duration (or pulse width) (measured inmicroseconds), and pulse rate (measured in pulses per second).

The stimulation circuitry 44 may, e.g., include a single current orvoltage source (not shown) for conveying stimulation energy to selectedones of the electrodes 28 as a group and returning the stimulationenergy to selecting ones of the electrodes as a group, or multiplecurrent or voltage sources (no shown) for independently conveyingstimulation energy to selected ones of the electrodes and independentlyreturning the stimulation energy to selected ones of the electrodes.

In any event, electrical stimulation will occur between two (or more)activated electrodes, one of which may be the case 34. Simulation energymay be transmitted to the tissue in a monopolar or multipolar (e.g.,bipolar, tripolar, etc.) fashion. Monopolar stimulation occurs whenselected ones of the lead electrodes 28 are activated along with thecase 34 of the microstimulator 12, so that stimulation energy istransmitted between the selected lead electrodes 28 and the case 34.Bipolar stimulation occurs when two of the lead electrodes 28 areactivated as anode and cathode, so that stimulation energy istransmitted between the selected electrodes 28. Tripolar stimulationoccurs when three of the lead electrodes 28 are activated, two as anodesand the remaining one as a cathode, or two as cathodes and the remainingone as an anode. Thus, when the microstimulator 12 is operated in amonopolar mode, the stimulation circuitry 44 will convey electricalstimulation energy to the lead electrodes 28 and return the electricalstimulation to the case 34. When the microstimulator 12 is operated in amultipolar mode, the stimulation circuitry 44 will convey electricalstimulation energy to lead electrodes 28 and return the electricalstimulation energy at different lead electrodes 28.

In the illustrated embodiment, when the microstimulator 12 is operatedin the monopolar mode, the electrical energy that is conveyed from themicrostimulator 12 to the target tissue site is cathodic (i.e., theactivated lead electrodes 28 are cathodes), and the electrical energyreturned to the case 34 of the microstimulator 12 is anodic (i.e., thecase 34 serves as an anode). However, the assignment of anodic andcathodic current to the lead electrodes 28 and case 34 will ultimatelydepend on the application in which the system 10 is intended to use.That is, if the nerve fibers at the target tissue site 32 are to bestimulated with cathodic current, the lead electrodes 28 will becathodes and the case 34 will be an anode. On the other hand, if thenerve fibers at the target tissue site 32 are to be stimulated withanodic current, the lead electrodes 28 will be anodes and the case 34will be a cathode. In any event, the lead electrodes 28 and the case 34will be oppositely polarized.

The stimulation energy may be delivered between electrodes as monophasicelectrical energy or multiphasic electrical energy. Monophasicelectrical energy includes a series of pulses that are either allpositive (anodic) or all negative (cathodic). Multiphasic electricalenergy includes a series of pulses that alternate between positive andnegative. For example, multiphasic electrical energy may include aseries of biphasic pulses, with each biphasic pulse including a cathodic(negative) stimulation pulse and an anodic (positive) recharge pulsethat is generated after the stimulation pulse to prevent direct currentcharge transfer through the tissue, thereby avoiding electrodedegradation and cell trauma. That is, charge is conveyed through theelectrode-tissue interface via current at an electrode during astimulation period (the length of the stimulation pulse), and thenpulled back off the electrode-tissue interface via an oppositelypolarized current at the same electrode during a recharge period (thelength of the recharge pulse).

In the illustrated embodiment, the microstimulator 12 isbattery-powered, and thus, the electronic circuitry 38 includes anenergy storage device 46; for example, a replenishable or rechargeablebattery, such as a lithium ion battery, an electrolytic capacitor, asuper- or ultra-capacitor, or the like). If the energy storage device 46is replenishable or rechargeable, an external charger (not shown) can beused to charge the power source via an inductive link. The energystorage device 46 is configured to output a voltage used to supply thevarious components of the electronic circuitry 38 with power. The energystorage device 46 also provides power for any stimulation currentgenerated by the stimulation circuitry 44.

The microstimulator 12 further comprises telemetry circuitry 48 (whichincludes an antenna in the form of an inductive coil and transceiver)for receiving programming data (including stimulation parameters),transmitting status data to and from the remote controller 16, andreceiving power from an external device, which may be the remotecontroller 16. The microstimulator 12 further comprises controlcircuitry 50 (which may be embodied in an integrated circuitry (IC)chip) for operating the stimulation circuitry 44 in accordance with aset or sets of stimulation parameters (e.g., selection of activatedelectrodes, pulse amplitude, pulse width, pulse rate, etc.), and amemory 52 for storing the stimulation parameters. The memory 52 may beany type of memory unit, such as, but not limited to, random accessmemory (RAM), status RAM (SRAM), EEPROM, a hard drive, or the like.Thus, the use of the control circuitry 50 and memory 52 allow thestimulation parameters to be adjusted to setting that are safe andefficacious with minimal discomfort for each individual. Specificstimulation parameters may provide therapeutic advantages for differentpatients or for various types and classes of ailments. For instance,some patients may respond favorably to intermittent stimulation, whileothers may require continuous stimulation for treatment and relief.

In addition, different stimulation parameters may have different effectson different tissue. Therefore, stimulation parameters may be chosen totarget specific neural or other cell populations and/or to excludeothers, or to increase activity in specific neural or other cellpopulations, and/or to decrease activity in others. For example, arelatively low pulse rate (i.e., less than 100 pulses per second (pps))may have an excitatory effect on surrounding neural tissue, leading toincreased neural activity (“excitatory stimulation”), whereas arelatively high pulse rate (i.e., greater than 100 pps) may have aninhibitory effect, leading to decreased neural activity (“inhibitorystimulation”). As another example, a relatively low pulse amplitude(typically less than 15 mA), but dependent on the distance betweenelectrodes and nerve fibers) are likely to recruit relatively largediameter fibers (e.g., A-α and/or A-β fibers), while not recruitingrelatively small diameter fibers (e.g., A-δ and/or C fibers). In theillustrated embodiment, the pulse rate may be in the range of 2-20 pps,the pulse duration may be in the range of 50-350 microseconds, and theamplitude may be in the range of 1-5V at about 1-50 mA.

Significant to the present inventions, the microstimulator 12 isdesigned in a manner that prevents, or at least reduces, the pocketstimulation phenomenon when operated in a monopolar mode.

For example, in the case where the nerve fibers stimulated in the tissuepocket 30 are smaller than the nerve fibers stimulated at the targettissue site 32 for therapy, stimulation energy having relatively shortpulse widths might be used to selectively stimulate the larger nervefibers at the target tissue site 32 without stimulating the smallernerve fibers at the tissue pocket 30. The difference between theamplitude at which smaller nerve fibers are stimulated and the amplitudeat which large nerve fibers are stimulated allows the amplitude of theelectrical energy to be adjusted within a usage range where the largernerve fibers at the target tissue site 32 are stimulated, while thesmaller nerve fibers at the tissue pocket 30 are not stimulated.Reducing the pulse width of the electrical stimulation effectivelyincreases this usage range.

The benefits of this technique can be better appreciated with referenceto FIG. 4, which illustrates the strength-duration curves for smallernerve fibers (solid line) and larger nerve fibers (dashed line).Notably, a strength-duration curve represents the pulse amplitude andpulse width needed to stimulate a nerve fiber of a specified diameter,and the usage range with respect to the strength-duration curve is thevariance from the strength-duration curve that maintains the stimulationenergy between the point at which it is perceived by the patient (i.e.,the point at which stimulation at the tissue target site 32 (shown inFIG. 1) is perceived) and the point at which it is uncomfortable for thepatient (i.e., the point at which stimulation at the tissue pocket 30 isperceived). It should be noted that at shorter pulse widths, the usagerange between large and small nerve fibers is greater (assuming a pulsewidth of 50 μs, the usage range is 60% of the amplitude that stimulatesthe large nerve fibers) than the usage range between larger and smallnerve fibers (assuming a pulse width of 250 μs, the usage range is 41%of the amplitude that stimulates the large nerve fibers). It followsfrom this that using stimulation energy with small pulse widths is moreselective, and therefore, can be more easily used to avoid stimulationof the relatively small nerve fibers in the tissue pocket 30 (shown inFIG. 1), while stimulation energy with larger pulse widths is not asselective, and therefore, more difficult to avoid stimulation of therelatively small nerve fibers in the tissue pocket 30.

In one exemplary manner that uses stimulation energy with smallerpulsewidths to avoid pocket stimulation, pulsed electrical energy isinitially conveyed from the microstimulator 12 to the lead electrodes28, thereby stimulating the target tissue site 32 remote from the tissuepocket 30. The initially conveyed pulsed electrical energy is thenreturned to the case 34 of the neurostimulator, potentially causingstimulation of the tissue pocket 30. If stimulation of the tissue pocket30 is detected (e.g., by the patient experiencing discomfort in thatregion and relaying this information to the clinician), the pulse widthof the stimulation energy is decreased (e.g., in the range of 10-100 μs,and preferably equal or less than 50 μs).

The pulsed electrical energy is subsequently conveyed from themicrostimulator 12 to the lead electrodes 28, thereby stimulating thetarget tissue site 32 again. The step of subsequently conveying thepulsed electrical energy may be performed by simply continuing theconveyance of the initially electrical energy or by ceasing conveyanceof the pulsed electrical energy and then again conveying the pulsedelectrical energy. In any event, the subsequently conveyed pulseelectrical energy is then returned to the case 34 of the microstimulator12 again. For the purposes of this specification, the term “initial”with reference to electrical energy conveyance does not necessarily meanthe first time the electrical energy is conveyed by the microstimulator12, but rather merely presupposes that there will be a subsequentconveyance of electrical energy.

Because the subsequently conveyed electrical energy is more selectivethan the initially conveyed electrical energy due to the decreased pulsewidth, stimulation of the tissue pocket 30 will be decreased, if noteliminated altogether. The pulse width of the stimulation energy can befurther decreased to prevent stimulation of the tissue pocket 30 if notalready eliminated. It should be noted that, as illustrated in thestrength-duration curves in FIG. 4, the point at which a nerve fiber ata specific size is stimulated increases as the pulse width of thestimulation energy decreases, thereby potentially losing stimulation ofthe target tissue site 32 if adjustments in the amplitude are not made.

As a result, it may be desirable to increase the amplitude of thestimulation energy as the pulse width of the stimulation energy isdecreased. Adjustment of the pulsewidth and amplitude of the stimulationenergy can be manually performed via operation of the remote controller16 or automatically performed, such as described in U.S. patentapplication Ser. No. 11/553,447, entitled “Method of MaintainingIntensity Output While Adjusting Pulse Width or Amplitude” and U.S.patent application Ser. No. 12/606,050 entitled “System and Method forAutomatically Adjusting Pulse Parameters to Selectively Activate NerveFibers,” which are expressly incorporated herein by reference.

As another example of preventing pocket stimulation, the microstimulator12 may be provided with spatially segmented electrodes (in this case,three electrodes E1-E3) that are associated with the case 34, as shownin FIG. 5. Although the case electrodes E1-E3 are illustrated ascompletely covering the outer surface of the case 34, the electrodesE1-E3 can be disposed on or form only a portion of the outer surface ofthe case 34. The microstimulator 12 is configured to independentlyreturn the pulsed electrical energy to the respective electrodes,thereby reducing or preventing inadvertent stimulation of the tissuepocket 30 (shown in FIG. 1).

The electrodes E1-E3 may form the structure of the case 34 itself, ormay be formed of electrically conductive and biocompatible materialmounted to the outside of the case 34. The shape of the electrodes 28may, e.g., be ring-shaped, as shown, or radial or disk-like. Ultimately,the shape of the electrodes 28 will depend upon the shape of the case34. Although three ring-shaped electrodes E1-E3 are shown to form thecase 34 illustrated in FIG. 5, a different number of electrodes withdifferent shapes can be used. For example, as shown in FIG. 7, thesame-shaped case 34 is used, but two electrodes E1-E2 can form the twohalves of the case 34 that are coupled together in a clam-shellarrangement.

In one technique for independently returning electrical energy to thecase electrodes 28, a temporally segmented current waveform can be used.In particular, the electrical pulses returned to the respectiveelectrodes 28 are temporally interleaved, as shown in FIG. 6. Forexample, for each stimulation pulse delivered to the lead electrodes 28,a pulse or a train of pulses can be returned to the first case electrodeE1, then a pulse or a train of pulses can be returned to the second caseelectrode E2, and then a pulse or a train of pulses can be returned tothe third case electrode E3. In this case, each return pulse necessarilyhas a shorter duration than the stimulation pulse. The pulses can besequentially returned to the electrodes E1-E3, as shown in FIG. 6, oralternatively, can be randomly returned to the electrodes E1-E3. Thisprocess can be repeated for the same stimulation pulse, such thatmultiple pulses can be returned, as illustrated in FIG. 6.

It can be appreciated that the foregoing technique minimizes temporalsummation of voltage at or across the neural membranes by virtue of thetemporal spacings of the return pulses, and minimizes spatial summationof the voltage at or across the neural membranes by virtue of spacing ofthe electrodes 28. In alternative embodiments, any of the leadelectrodes 28, in combination with the case electrodes 28, can be usedto return the electrical energy.

In another technique for independently returning electrical energy tothe case electrodes, the case electrodes are selectively activated; forexample, via operation of the remote controller 16, thereby changing theelectrical field within the tissue pocket 30 (shown in FIG. 1). Thus,the combination of case electrodes that reduces or eliminates the pocketstimulation can be selected. For example, electrical energy can beconveyed to the lead electrodes 28 and returned to the case 34 fordifferent combinations of activated case electrodes. Based on patientfeedback, the combination of case electrodes with the best result canthen be selected as the return electrodes.

If the stimulation circuitry 44 of the microstimulator 12 (shown in FIG.3) only comprises a single current or voltage source, the relativecurrents returned to the activated case electrodes cannot be controlled.However, if multiple current or voltage sources are provided, thefractionalized currents returned to the activated case electrodes can becontrolled. For example, as shown in FIG. 7, one case electrode E1 canreturn 65% of the current, and another case electrode E2 can return 35%of the current.

In alternative embodiments, selected ones of the lead electrodes 28, incombination with the activated case electrodes E1-E2, can be used toreturn the electrical energy. In this case, only a portion of theelectrical energy is returned to the case electrodes E1-E2. For example,the return current can be split 55%, 35%, and 10% between a selectedcase electrode E1, another selected case electrode E2, and a selectedlead electrode 28, respectively.

As another example of preventing pocket stimulation, segmentedelectrodes can be associated with the case 34 of the microstimulator 12in the same manner described above with respect to FIGS. 5 and 7. Inthis case, however, four electrodes are provided, as shown in FIG. 8,with electrode E1 forming the main body of the case 34, electrode E2forming the bottom edge of the case 34 that meets the connector 36,electrode E3 forming the top edge of the case 34, and electrode E4forming the flat top surface of the case 34.

Because geometrical features, such as corners and edges (e.g., theelectrodes E2 and E3), are known to exhibit higher current densities asopposed to other geometrical features, such as smooth or flat surfaces(e.g., the electrodes E1 and E4), these higher current density regionsmay be more likely to cause stimulation than the other regions of thecase 34. Thus, the current densities on the case electrodes E1-E4 aremade as uniform as possible by decreasing the variances of the currentdensities to a relatively low value, (e.g., 25% or less, and preferably10% or less). As a result, the maximum current density on the caseelectrodes E1-E4 is minimized, thereby minimizing or completelyeliminating pocket stimulation.

The current densities on the case electrodes E1-E4 can be made moreuniform by varying the relative impedances of the case electrodes E1-E4,with the electrodes that normally have higher current densities (e.g.,electrodes having edges and corners, such as electrodes E2 and E3)having a relatively higher impedance, and the electrodes that normallyhave lower current densities (e.g., the electrodes having smooth or flatsurfaces, such as electrodes E1 and E4) having a relatively lowerimpedance. For example, FIG. 8 illustrates a circuit representation ofthe case electrodes E1-E4 and associated impedances R1-R4. As thereshown, the input terminal 42 of the stimulation circuitry 44 (shown inFIG. 3) is coupled in parallel to the four case electrodes E1-E4. Itshould be noted, however, that the input terminal 42 can alternativelybe coupled in series to the four case electrodes E1-E4.

The relative impedances of the case electrodes E1-E4 can be changed,e.g., by making the electrodes from materials with differentresistivities. For example, the edge electrodes E2, E3 may be composedof a material that has a higher resistivity than that of the flatterbody and top electrodes E1, E4, thereby decreasing the current densityexhibited by the edge electrodes E2, E3 relative to the current densitythat they would have exhibited had they been made of the same materialas the body and top electrodes E1, E4.

As another example, passive components (not shown), such as resistors,can be located between the input terminal 42 of the stimulationcircuitry 44 and the particular case electrodes, such as the edgeelectrodes E2, E3, with the anticipated higher current densities. Inthis manner, the passive components decrease the current density on theedge electrodes E2, E3 relative to the current density that would havebeen on the electrodes E2, E3 absent the passive components.Alternatively, passive components can be located between the inputterminal 42 of the stimulation circuitry 44 and all four case electrodesE1-E4, with the combined resistance value of the passive component orcomponents associated with each of the electrodes increasing with theanticipated current density.

Although the case electrodes E1-E4 have been described as being coupledto a single input terminal 42, in alternative embodiments, the caseelectrodes E1-E4 can be respectively coupled to a plurality of inputterminals. In this case, if the stimulation circuitry 44 comprisesmultiple current or voltage sources, the electrical currents at therespective input terminals can be independently adjusted by increasingor decreasing the current or voltage of the sources. For example, forthe edge electrodes E2, E3 (i.e., the case electrodes where a highcurrent density is anticipated), the electrical current at therespective input terminals coupled to these electrodes can be set to berelatively low, whereas for the flat or smooth electrodes E1, E4 (i.e.,the case electrodes where a low current density is anticipated), theelectrical current at the respective input terminals coupled to theseelectrodes can be set to be relatively high.

As another example of preventing pocket stimulation, the current densityon the case electrode(s) can be reduced by using an external lead withadditional electrodes that are not intended for stimulation therapy. Inthis case, the electrical energy is conveyed from the microstimulator 12to the lead electrodes 28, thereby stimulating the target tissue site 32remote from the tissue pocket 30 (shown in FIG. 1), while the conveyedelectrical energy is returned to both the case electrode(s) and theexternal lead.

For example, in one embodiment illustrated in FIG. 9A, the stimulationlead 14, itself, serves as the external lead, with additional electrodes50 located on the proximal end of the stimulation lead for returning theelectrical energy along with the case electrode(s). In anotherembodiment illustrated in FIG. 9B, an external lead 52 that is separatefrom the stimulation lead 14 is provided. In still another embodiment,an external lead 54 in the form of an electrical conductor is used toreturn the electrical energy along with the case electrode(s). Theelectrical conductor 54 could, e.g., be composed of a flexible or rigidhypotube or wire composed of a biocompatical, electrically conductive,material, such as stainless steel or platinum. If the electricalconductor 54 is made from a hypotube, it can be etched for flexibility.

In either of the embodiments in FIGS. 9A-9C, the additional electrodes50 can be directly electrically coupled to the case 34 via the connector36, thereby reducing the current density on the case electrode(s). Forthe purposes of this specification, an element is directly electricallycoupled to another element if no active components are located betweenthe elements. In the embodiments of FIGS. 9A and 9B, the electrodes 50can be ganged together using a common lead conductor (not shown).Alternatively, any of the additional electrodes 50 can be electricallycoupled to input terminals of the stimulation circuitry 42 that are notalready used to return electrical energy from the case electrodes. Inthis case, selected ones of the additional electrodes 50 may beactivated in the case where the stimulation circuitry 42 has a switchand/or fractionalized currents can be assigned to the activatedelectrodes 50 in the case where the stimulation circuitry 42 includesmultiple current or voltage sources.

As another example of preventing pocket stimulation, the current densityon the case electrode(s) can be reduced by using an external lead withadditional electrodes that are not intended for stimulation therapy. Inthis case, the electrical energy is conveyed from the microstimulator 12to the lead electrodes 28, thereby stimulating the target tissue site 32remote from the tissue pocket 30 (shown in FIG. 1), while the conveyedelectrical energy is returned to both the case electrode(s) and theexternal lead.

As another example of preventing pocket stimulation, the current densityon the case electrode(s) can be reduced by using an expandable electrodethat, like the previous embodiment, is not intended for stimulationtherapy. In this case, the electrical energy is conveyed from themicrostimulator 12 to the lead electrodes 28, thereby stimulating thetarget tissue site 32 remote from the tissue pocket 30 (shown in FIG.1), while the conveyed electrical energy is returned to both the caseelectrode(s) and the expandable electrode.

For example, in one embodiment illustrated in FIG. 10A, an expandableelectrode 56 is directly electrically coupled to the top of the case 34via a conductor 58, and is therefore used to return electrical energyalong with the case electrode(s). To allow for its expansion, theelectrode 56 includes a plurality of stacked blades 58 that are tiedtogether at one end. When the electrode 56 is in a non-expandedgeometry, the stacked blades 58 are folded together (left side of FIG.10A), so that it can be easily introduced into the patient's body duringimplantation. To place the electrode 56 in an expanded geometry, thestacked blades 58 are radially folded out, much like a fan (right sideof FIG. 10A), thereby increasing the surface area of the electrode 56after implantation. In the illustrated embodiment, the shape of theblades 58 is rectangular, although other shapes may be used, includingthose with rounded edges or triangular shapes.

In another embodiment illustrated in FIG. 10B, an expandable electrode60 is directly electrically coupled to the top of the case 34 via aconductor 62, and is therefore used to return electrical energy alongwith the case electrode(s). To allow for its expansion, the electrode 60includes a plurality of stacked blades 62 much like the expandableelectrode 56 described above, except that the stacked blades 62 are tiedto each other at both ends. When the electrode 60 is in a non-expandedgeometry, the stacked blades 62 are folded together (left side of FIG.10B), so that it can be easily introduced into the patient's body duringimplantation. To place the electrode 60 is in an expanded geometry, thestacked blades 62 are laterally folded out, much like blinders (rightside of FIG. 10B), thereby increasing the surface area of the electrode60 after implantation. In the illustrated embodiment, the shape of theblades 62 is rectangular, although other shapes may be used, includingthose with rounded edges or triangular shapes.

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

1. A method of providing therapy using a neurostimulator implantedwithin a tissue pocket of a patient, the neurostimulator having a caseand one or more electrodes disposed on the case, comprising: conveyingelectrical energy from the neurostimulator, thereby stimulating a targettissue site remote from the tissue pocket; and returning the electricalenergy to the one or more electrodes and at least one external leadconnected to the case.
 2. The method of claim 1, wherein the one or moreelectrodes forms at least a portion of the case.
 3. The method of claim1, wherein the at least one external lead is electrically coupleddirectly to at least one of the one or more electrodes.
 4. The method ofclaim 1, wherein the electrical energy is returned to the one or moreelectrodes and the at least one external lead to prevent inadvertentstimulation of the tissue pocket that would otherwise occur.
 5. Themethod of claim 1, wherein the electrical energy is conveyed from theneurostimulator to at least one stimulation lead connected to theneurostimulator to stimulate the target tissue site.
 6. The method ofclaim 5, wherein the at least one stimulation lead comprises the atleast one external lead.
 7. The method of claim 5, wherein the at leastone stimulation lead is separate from the at least one external lead. 8.The method of claim 1, wherein the at least one external lead comprisesa plurality of electrodes, the method further comprising selectivelyactivating ones of the plurality of electrodes that will return theelectrical energy.
 9. The method of claim 8, further comprisingindependently adjusting the electrical energy returned to the activatedelectrodes.
 10. The method of claim 9, wherein independently adjustingthe electrical energy returned to the activated electrodes comprisesselecting a fractionalized electrical current for the activatedelectrodes.
 11. The method of claim 1, wherein the at least one externallead comprises an expandable electrode that will return to theelectrical energy, the method further comprising expanding theexpandable electrode to increase a surface area of the electrode, andimplanting the expanded electrode into the patient.
 12. Aneurostimulator, comprising: a case; one or more electrodes associatedwith the case; one or more connectors configured for being coupled toone or more stimulation leads; at least one external lead electricallycoupled directly to at least one of the one or more electrodes; andstimulation circuitry contained within the case, the stimulationcircuitry configured for conveying electrical energy to the one or morestimulation leads, and returning the electrical energy to the one ormore electrodes and the at least one external lead.
 13. Theneurostimulator of claim 12, wherein the one or more electrodes form atleast a portion of the case.
 14. The neurostimulator of claim 12,wherein the at least one stimulation lead comprises the at least oneexternal lead.
 15. The neurostimulator of claim 12, wherein the at leastone stimulation lead is separate from the at least one external lead.16. The neurostimulator of claim 12, wherein the at least one externallead comprises an expandable electrode configured to return theelectrical energy, the expandable electrode configured to be manipulatedto increase a surface area of the expandable electrode.
 17. Theneurostimulator of claim 16, wherein the expandable electrode comprisesa plurality of stacked blades.
 18. The neurostimulator of claim 16,wherein the expandable electrode is expandable in the radial direction.19. The neurostimulator of claim 16, wherein the expandable electrode isexpandable in the lateral direction.